Small model engines are composed mainly of brass castings and of steel which requires no special forging for the purpose. The screws or bolts used to unite the parts are usually purchased in a finished state. Makers of these employ machinery which acts almost automatically, and the screws are sold at a very cheap rate. Larger models require special forgings for the crank shaft, and the castings employed are of iron, which is considerably cheaper than brass.
The castings are made from patterns which are counterparts of the object required. These are imbedded in sand, and leave a matrix, into which molten metal is poured, producing, on solidifying, a facsimile of the pattern. The operation is always carried out in a foundry, where the necessary furnaces and moulding appliances are at hand. The founders charge for the rough castings by weight, and they cost merely a trifle over the value of the metal. It is, however, necessary to supply the requisite patterns before a founder can proceed to do his part of the work.
All vendors of castings have patterns from which their castings are moulded, and of course they charge, in addition to a profit on the cost of the metal, something for the use of the patterns. The patterns for a founder’s use require certain modifications, which it is unnecessary to explain in detail. Some are made in two or more parts, with pins to hold them together. Some have projections affixed to them; these make prints in the mould to receive cores, which form holes in the casting. Those patterns which enter deeply into the moulding sand are made tapering, to draw out easily. In all cases they must be made sufficiently large to allow for shrinkage in the metal. Ordinary iron castings shrink about one-eighth of an inch to the foot; brass about half as much again. Pattern-makers use a ‘contraction-rule’ to work by; this is made longer than the standard measurement, and patterns made according to it are the correct size to allow for shrinkage.
From what has just been said it will be readily understood that vendors of castings charge various prices for their goods. Nor in every case is the quality in accordance with the price, and it is difficult to give the exact prices that should be paid for good castings. Speaking generally, the price is regulated by the weight, and the rate per pound is decided by the seller. In the catalogues issued by various firms will be found the prices charged. As an example of the difference, I notice that a certain size of bolts made by one firm are retailed by shopkeepers at rates varying from 33 to 200 per cent. profit; the same rule probably holds good in all other items.
Those readers who are not possessed of a lathe will not have the means of finishing the cylinders and some other parts which have to be turned. These can, however, be bought in various stages of completion, and the beginner who has only a screwdriver may now purchase the component parts, and, having screwed his engine together, he may claim some merit for his share in the erecting department.
Sets of castings quite finished and ready to be screwed together are now sold. These are generally of the cheaper class, and, tacked on cards, may be seen in the windows of opticians. The prices for the complete engine, with boiler, lamp, and all other parts, range from about five shillings upwards. A few words on the better type of partially finished parts.
These castings are more expensive than those quite rough, but they afford an opportunity of displaying considerable skill and judgment in completing them.
Boring the cylinders is the operation most likely to baffle the tyro. This is done by vendors of castings for about two shillings and sixpence for cylinders 1-in. bore. This charge includes turning the flanges ready to receive the covers, and also boring the steam-ways and cutting the port-holes. When all this has been done it will be necessary to use a lathe to turn the covers for the cylinder, and also for making the piston. The cylinder may be purchased complete with the covers screwed on and the slide-valve fitted. One an inch in the bore costs half a guinea. Every piece of an engine may be bought separately in a finished state, so that they only require putting together, and when the young engineer has not the requisite tools for doing the work his best plan will be to purchase the finished parts.
A glance at an engine will show that nearly every part of it has been fashioned on a lathe. This tool is indispensable for all kinds of engineering work, but as it is somewhat costly it frequently occurs that tyros are compelled to forego its ownership and get the necessary turning executed by a professional latheman. Those readers who are happily possessed of the king of tools—or the father of mechanism, as the lathe has been aptly dubbed—will have the advantage of being able themselves to execute the work throughout.